For casual rounds I usually carry 10 in a Fade Lite. If I'm going to a course I don't know or into a tournament I add up to 10 more and carry an Innova Competition. Those "10 more" are mostly backups or discs that probably won't see any action anyway though. My current setup for casual rounds is:
Putters: Anode, Ion
Mids: Buzzz, Fuse, Pain
Fairways/Drivers: Trident, Saint, Avenger SS, Gazelle, Predator
Two of the last three best rounds I've thrown over the last month have come with very minimal disc use. One was just Buzzz and Anode, and the other was Gazelle, Buzzz, and Anode. I've found that the less complex I make it, the better.
IF I were to carry a bigger bag, it would be something like this:
Putters: Anode, Ion, Pure x2
Mids: Buzzz x3, Comet/Fuse, Pain
Everything else: Trident, Saint/Volt, Avenger SS, Gazelle x5, Predator/Firebird, Wraith
But again... If I can throw a solid round with two or three discs, why haul around 20?